The majority of particulate organic matter (POM) transported through river networks is <1 mm in diameter and represents approximately 40% of its total mass flux. However, fluvial research to date has mostly focused on the processing of leaf litter and dissolved organic matter whereas processing of suspended fine POM is poorly understood in river networks. Therefore, we conducted field samplings to examine biochemical composition and microbial decomposition among different size classes along the fine POM size continuum and between mountainous and agricultural streams. We collected water samples from five streams dominated by agricultural land use and five streams dominated by mountainous areas, and separated the fine particles into five size classes (500-100, 100-50, 50-15, 15-2.7, and 2.7-0.7 µm). In each size class, we measured bacterial production, C:N ratios, and fatty acid composition of the collected particles. Our results show that particles <15 µm have highest bacterial production rates whereas particles between 15 to 500 µm had ~3000 times lower rates per volume. In addition, bacterial production rates on fine particles from agricultural streams showed higher activities compared to mountainous streams in all size classes. The fatty acid (FA) composition did not differ between mountainous and agricultural streams but among the size classes. Particles <15 µm were colonized by bacterial FA whereas larger particles mostly contained algal FA. Hence, fine particles <15 µm are hot spots of carbon turnover in streams and understanding their turnover and drivers is necessary to better understand the fluvial carbon cycle.

Freshwater pools are a characteristic feature of northern peatlands, often forming complex systems, with widely differing pool morphologies. These pools receive large inputs of dissolved organic carbon (DOC) from the surrounding terrestrial environment and, as a result of biogeochemical processing within the pools, have been found to be net emitters of greenhouse gases (GHGs). The GHG fluxes from peatland pools are known to vary diurnally, however the magnitude of these variations and the drivers behind them remain poorly understood. Previous studies have mainly involved daytime only measurements in the summer season, primarily driven by logistical necessity. This study examined diurnal variations in CO₂ and CH₄ fluxes from a selection of peatland pools in the blanket bog of the Flow Country, Europe’s largest blanket bog, with sampling conducted across different seasons and including different pool size categories. The results suggest that there is notable variability in the fluxes from peatland pools across daily cycles. In general, daytime only measurements underestimated actual CO₂ fluxes (daytime measurements were up to 10 times lower and in some cases showed CO₂ uptake, while the daily average was an emission), while overestimating CH₄ fluxes (by up to 2.1 times). Further variability was found seasonally and there were clear differences found between different pool sizes. Monthly spot measurements at similar pools in the Flow Country were used to investigate the implications of these findings for calculating annual flux values from peatland pools, further highlighting the importance of including temporal variability when modelling the carbon balance of peatlands.

River Ecosystem Metabolism (REM) represents a cornerstone for river ecology and freshwater management as it includes the total interrelated fluxes that fix and mineralize organic carbon in an ecosystem and integrates the response to a broad range of natural and human factors. The proliferation of reliable sensors, and easy-to-use software for the calculation of Gross Primary Production (GPP) and Ecosystem Respiration (ER) represents an excellent opportunity to gain understanding of the river ecosystem energetics. However, to date very few studies have focused on analysing multiannual regimes of REM and their environmental drivers from a large-scale perspective. Moreover, the application of classification approaches to develop functional typologies can be very valuable to synthesized and understand the complex REM patterns at these spatio-temporal scales. In this study we estimated the GPP, ER and REM regimes of 49 Iberian rivers and developed a data-driven classification, based on a set of indices accounting for the mean and extreme values of GPP and ER. We found 3 and 4 GPP and ER types, respectively, distinguished by their monthly magnitude and seasonality. Our results suggested a clear presence of characteristic functional typologies in the Iberian rivers driven by a consistent set of natural gradients, and human related drivers (i.e. flow alteration and land cover and land use). This work has important implications since functional typologies could be linked to certain river ecosystem services rates, and they could aid to understand global carbon dynamics and predict the consequences of large-scale disturbances such as climatic change.

Urban freshwaters favour high rates of microbial CO2 and CH4 production. As urbanisation increases globally, a rapidly growing number of freshwater bodies are subject to these conditions. Despite this, no studies have investigated CO2 and CH4 emissions at a sufficient temporal resolution to accurately characterise the dynamics of the system. To start bridging this gap, we connected automatically ventilating floating chambers (AFCs) with a high-resolution Cavity Ring-Down Spectroscopy (CRDS) analyser and deployed them in seven urban ponds for 24-hour periods . The unprecedented temporal resolution of the measurements revealed a clear diel influence on emissions rates, with average day-time CH4 fluxes 1.8 times higher than night-time fluxes, and large variability over 24 hours. Deployment of the AFCs in four seasons between autumn 2021 and summer 2022 also showed seasonal variation in flux rate, with CH4 efflux and CO2 influx increasing from winter to spring, then peaking in summer, and the prevailing pathway varying from near complete diffusion in the winter to strong ebullition dominance in the summer. High spatial variation both among and within ponds was driven by ebullition events. Collectively, these results underline the importance of high-resolution flux measurements and, most notably, that urban ponds are disproportionately large sources of CH4. and This calls for increasing efforts to capture diel and seasonal CH4 dynamics in urban freshwaters, particularly in light of ongoing global warming and urbanization trends.

The hydrological patterns of Mediterranean wetlands are variable and fluxes of greenhouse gases (GHGs) may vary accordingly. This poses the question of identifying appropriate methodologies and compartments to assess these fluxes. CO2 and CH4 fluxes were measured at different phases of the hydroperiod in representative Mediterranean wetlands and shallow lakes from the Iberian Peninsula. The survey comprised both coastal and inland sites, either saline or freshwater. Comparison among sites showed organic matter content in sediments as a main factor explaining GHGs production. Concomitantly, temperature and salinity showed an opposite effect, particularly for CH4, being this direct or inverse respectively. Strong differences were observed along the flooding gradients within wetlands, with a higher carbon outflow in shallower zones compared to the deepest ones, still CH4 emissions were usually highest at the intermediate depths, which are frequently flooded but still very shallow. We complementary performed ex-situ medium-term and in-situ short-term incubations using sediment cores and closed chambers, respectively. Because the longer incubation time, sediment cores integrated better the CH4 diffusion and ebullition processes, although the closed chambers also allowed to monitor them. These findings highlight the key role of hydrological dynamics explaining GHG fluxes and can serve to inform monitoring design for estimating carbon budgets, focusing on the ways that it can be either underestimated or overestimated. This work is supported by projects CLIMAWET-CONS (PID2019-104742RB-I00), funded by Agencia Estatal de Investigación and the Ministerio de Ciencia e Innovación (Gobierno de España), and Wetlands4Climate (LIFE19 CCM/ES/001235), funded by the EU-LIFE programme.

Over the past two centuries, upland lake and stream ecosystems across the UK became acidified by air pollutants derived from fossil fuel-based emissions of sulphur, nitrogen and hydrogen chloride to the atmosphere. Since the 1980s, a series of international agreements aimed at protecting sensitive environments, led by the United Nations Economic Council for Europe, and the European Union, have resulted in a major reduction in emissions, and consequently acid deposition. The UK Acid Waters Monitoring Network, later to become the Upland Waters Monitoring Network (UWMN), was established to audit the efficacy of emission controls with regard to the health of upland freshwater ecosystems. In this presentation we summarise the results of a recent analysis of the first three decades of UWMN water chemistry data in relation to changes in acid deposition. We show that these environments have been undergoing profound chemical change as a direct consequence of the air policy measures, involving not only reductions in water acidity but also substantial increases in dissolved organic matter that have major potential consequences for aquatic primary productivity and the management of the UK’s upland drinking water supplies. As the deposition of acid pollutants begins to level off, concerns remain over the capacity of historically heavily acidified catchment soils to enable full recovery of the receiving waters, the potential longer-term legacy of reactive nitrogen deposition, and the extent to which projected changes in regional air temperatures and precipitation patterns will further influence these systems into the future.

It is now recognised that freshwaters are active components of the global carbon cycle; rivers and lakes process the organic matter and nutrients they receive from their catchments, emit carbon dioxide and methane, sequester carbon dioxide through aquatic primary production, and bury carbon in their sediments. Human activities have greatly modified natural aquatic biogeochemical processes and in some inland waters, this has led to large greenhouse gas emissions to the atmosphere. However these emissions are highly variable in time and space, occur via a range of pathways, and are consequently exceptionally hard to measure on the scales required. In the GHG-Aqua project we are establishing an integrated, UK-wide system for measuring aquatic GHG emissions, combining a highly instrumented sentinel sites with a distributed, community-run network of low-cost sensors systems deployed across inland waters to measure emissions from rivers, lakes, ponds, canals and reservoirs across key environmental gradients. In this presentation, we will describe the network and highlight some of our early findings. Using data from the UK’s first eddy covariance flux towers installed to quantify freshwater carbon dioxide and methane emissions, we show that overturning creates hot moments for methane emissions from lakes. High temporal resolution flux data from our sentinel sites also demonstrate seasonal variation in the source-sink potential of freshwaters. Insights from this globally unique, integrated measurement system will transform our capability to understand and quantify aquatic GHG emissions from inland waters.

In freshwater temporary systems, wet conditions are known to enhance both carbon burial and methane (CH₄) emissions. On the opposite, carbon dioxide (CO₂) emissions are expected to be enhanced during the phase of exposition to the air (dry period), thanks to aerobic conditions in the sediment. The balance between the two processes is fundamental to grasp the ultimate carbon budget of inland waters. This is especially true for groundwater-fed ecosystems whose water level depends on the fluctuation of the aquifers, such as temporary ponds.
We here present the preliminary results of an ongoing study performed on natural temporary ponds of the Landes de Gascogne (South-West of France). Here, about two thousand oligotrophic ponds develop on a sandy substrate within a landscape impacted by intensive crop farming and forestry. Global warming is expected to lengthen and intensify the drying up of these environments, and to diminish the capacity of carbon storage in sediments. With the aim of obtaining a multiseasonal carbon budget in function of the hydrological cycle, CO₂ and CH₄ fluxes are measured on a monthly basis on a set of temporary ponds. Floating and benthic chambers are deployed on submerged surfaces as well as on sediments exposed to the air. Concomitantly, a fine-scale bathymetry is realised and coupled to high-frequency measurements of the water level through automatic probes, in order to reconstruct the exact water level oscillation throughout the year. Also, the carbon storage in the sediments is assessed by sediment cores and stable isotope analysis.

Greenhouse gas (GHG) emissions from reservoirs are quantitatively relevant for atmospheric climatic forcing. These emissions have a large temporal variability, with daily changes accounting for a significant part of the total variability. However, most estimations for GHG fluxes are based on the upscaling of discrete measurements performed at daytime, and, in general, they did not account for the nighttime emissions. Here, we explored the daily patterns of CO2, N2O, and diffusive and ebullitive CH4 fluxes in two eutrophic reservoirs with contrasted morphometries in two different years. We found daily patterns for CO2, N2O, and diffusive CH4 fluxes with consistent higher emissions at daytime than at the nighttime irrespectively of reservoir morphometry. These three diffusive fluxes showed evident daily synchrony suggesting a common driver. The emissions were coupled with the daily solar cycle, wind speed, and water temperature. The daily emissions of the CO2, N2O, and CH4 were also positive and significantly related to oxygen saturation. In contrast, we did not find a consistent daily pattern for the ebullitive CH4 fluxes, although they represented a significant fraction of the total CH4 emitted in these reservoirs. Our study suggests that the daily variability in GHG emissions may be as relevant as the variability at spatial or inter-system variability and solar radiation increases emissions. Therefore, daily ranges should be considered in future GHG budgets to improve global estimates of GHG emissions from reservoirs.